49 



Mr. S. H. Enimens has tabulated the following steps in the oxidation 

 of pyrite.* Part of the sulphur of the pyrite is converted by the 

 oxygen and the moisture of the atmosphere into sulphuric acid, leaving 

 a residue of iron monosulphide. This is then attacked by the sulphuric 

 acid and ferrous sulphate results, while at the same time sulphuretted 

 hvdroeen is evolved. The reactions are as follows:! 



(7.) Fe S, 3 + H 2 = Fe S 4- H,S 4 . 

 (2.) FeS- H, S<V=FeS0 4 - H,S. 



If the sulphuretted hydrogen comes in contact — as it naturally must 

 in passing through the rock — with oxydizing pyrite, and if, as Emmens 

 holds, sulphurous anhydrite (S 2 ) is formed, together with the sulphuric 

 acid, the hydrogen sulphide will react with the sulphurous anhydrite 

 and form water and free sulphur. The reactions would be tabulated 

 thus : 



a. Fe S, - 0, = Fe S + S 2 . 



b. S 2 + 2 H 2 S = 2 H 2 - 3 S. 



or, as given by Emmens : 



(3.) Fe S, - 2 — 2 H 2 S = Fe S - 2 H 2 - 3 S. 



This sulphur maybe in part deposited, and in part again oxidized to 

 sulphuric acid, thus:$ 



(4.) S + 3 + H 2 = H 2 S0 4 

 this latter again attacking the monosulphide (Fe S). 



The third and fourth reactions probably do not take place in these 

 shales, the hydrogen sulphide being directly decomposed by the atmos- 

 phere, with the formation of sulphur and water, the former being 

 deposited where the oxidation takes place. Thus: 



2 H 2 S 4 0, = 2 H 2 + S 2 . 



The ferrous sulphate will absorb oxygen, and sulphuric acid, if the 

 latter is in excess, and form ferric sulphate, according to the following 

 reactions :§ 



(5.) 2FeS0 4 -0- H 2 S t = Fe 2 (S 4 ) 3 - H,0 



which would be the final result of the oxydatiou. But if the amount of 

 sulphuric acid is insufficient, or if the ferrous sulphate is carried in 

 solution and spread over the surface of the shales, it will oxidize in part 

 to ferric hydrate or limonite, which stains the shales. The reaction, 

 according to Emmens, is : 



Fe S i + 6 0. + H,0 = 4 Fe.,(S 4 ) 3 2 Fc,O s . H.,0. 



♦Stephen H. Emmens: '-The Chemistry of Gossan," Engineering and Mining 

 Journal, Dec. 17, 1892, p. 582, 

 iJEmmens, loc. cit. 

 JLoc. cit. 



